Stabilization of nitrocellulose



MV 3 94- R. E. REEVES Erm., l 2,404

` STABILIZATION OF NI'.IROCI'BI'JLULOSE` Filed May 5, 1944 Ely.; El

Mg so4l Pm a mmocELLuLos l0 2O 3o 4o 5o STAEILITY (immun-:5) 3&3 ad

l l amm/m Patented July 30, 1946 STABILIZATION F NITROCELLULOSE Richard E. Reeves and Joel E. Giddens, New O rl leans, La., assignors to United States of America as represented by Claude R. Wickard, Secretary of Agriculture, and his successors in of` Application May 5, 1944, serial No. 534,370.

2 claims. (01260-223) (Granted under the act of March 3, 1883, as l. amended April 30, 1928; 370 0. G. 757) V l This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described, if patented, may be manufactured and used by or `for the Government of the United States of America for governmental purposes without the payment tous of any royalty thereon.

This invention relates to nitrated cellulose, and has for its primary objects an improvement in the process of stabilizing such cellulose, and such other objects as will be apparent in consideration of the following specification:

Due to the use of sulfuric acid in the preparation of nitrocellulose, it is ordinarily necessary to remove the retained sulfate to stabilize the product. This is conventionally accomplished by repeated boiling in water and beating treatments, a long and tedious process. It is also known that addition of a little ammonia to the boiling water aids in stabilization. However, in this event it is necessary to control the amount. of ammonia added very closely, as the addition of ammonia little beyond slight alkalinity deleteriously affects the nitrocellulose.

According to this invention, ammonia isA used at a lower temperature, preferably room temperature, as one treatment, in which event the alkalinity need not be so carefully controlled.` Also.4

ammonium salts may be used instead of the hydroxide, andif the treating solution is buffered to remain slightly acidic, excess salt can be used with a hot solution with no deleterious eiect on the nitrocellulose, and thus be introduced in connection with a boiling treatment as claimed in copending application, Serial No. 670,086, filed May 16, 1946.

. Preferably, the ammonia treatment is combined with at least aprior boil, and boiling and beating treatments may be advantageously employed subsequent to the boil but prior to the ammonia treatment.

Room temperatures are most satisfactory for the ammonia treatment because of convenience, althoughhigher and lower temperatures are effective. In general; it may be said, however, that the higher the temperature the more closely the alkalinity must be controlled.

The` process has been carried out principally using cut cotton as the cellulosic raw material, but has been foundto be equally effective with cotton linters, and is also effective with nitrocellulose derived fromwood pulp. l The process is more specifically exhibited "nf, following examples: A EXAMPLEI Cotton fibers, cut to about 0.15 inch and purif, ed to meet the specifications for linters to be', used in the productionf-offpyro. nitrocellulose' by the Naval Powder Factory, were dried over@v night at C. A `32 g.- portion oftheldried iiber was stirred in a solutionconsisting of,v 231i g.,

of H2O, '866 g. 0fA H2SO4, and 494 g. QHNOS, all

chemicals used beingvof C rlP. or reagent grade. The mixture wask maintained at 40 C. for 2Q minutes,v immediately after which the formed nitrocellulose was ltered and drowned in cold tap water, It was then washed with 1l) changes of water, and was boiled f or 4 hours underreux and dried, giving a lot having a nitrogen cont The boiled nitrocelluloseas. above, obtained. was divided in four parts, one vof which was rinsed and dried and kept as a controlsample. The other threek parts were separately stirred. for..1 5 minutes, each with 50 parts o f ammonium hy@ droxide solution containing 0.05%, 0.20% and 1.00% of ammonia, respectively, at roomtempera-- turesto obtain three separate stabilized samples'.v The excess ammonium hydroxide was removed Tests were run on these samples against-the control sample using the known134.5 heat'test (Military explosives, War Department TechnicaA Manual 9-2900. August 29, 1940, DD. 66 and'l), and also using a `second procedure'hereinafter designated as the test. This second precedure was developed especially to handle small samples with safety and reproducibility, and is illustrated by the apparatusshown in Figure 1 of the drawing. In thisapparatus, -.a.suitable quantity (0.5 g. with an apparatus having dimens sions as shown in the drawing) ofthe sample to be tested is placed in a U-tubell immersed -ina tricresyl phosphate bath fl! maintained at 110i0.55 C. 'The U-tube IB isstoppered at its` upper ends, and has one of its legs connected by a tube l2 to one leg of a second U-tube I3. An indicator consisting of l ml. of an aqueous solution of 0.5% potassium iodate, 1.0% potassium iodide, and 3 drops of a 2% starch solution is placed in the second U-tube I3. This U-tube is also stoppered at its upper ends, and its other leg is connected through a tube I4 to a suitable suction device (not shown). Outside air is drawn from a tube I5 at the rate of 30 ml. per minute through sulfuric acid in flask I6, thence through soda line in bulb I'I, thence passing through U-tube IIL sweeping through the sample therein, and thence passing through tube I3, bubbling through the indicator therein. The air sweeps the volatile decomposition products from the sample, the end of the test being determined when` blue coloration develops in the` indicator. The time required for this coloration to'occur is taken as a measure of the stability of the sample tested With the 110 test, as above described, the stability of highly unstable nitrocellulose samples may be evaluated with comparative safety and the reproducibility with such samples is excellent. Moisture control of the sample is not such a critical factor as in the case of those tests which keep evolved gases in contact with the heated sample, and therefore the 110 test may be used with air-dried samples.

The 110 test and the 134.5 heat test often vyield approximately the same times for a given sample. In both tests samples showing times of less than minutes may be classified as having poor stability, those showing from 10 minutes to 25 minutes as having moderate stability, and

`those showing more than 25 minutes as having good stability.

The following table indicates results of the tests of the control and the three samples of Example I, the several results indicated from each test of a sample being obtained by repeated tests on the same sample.

Table 1 134:.5x heat 110 test Treatment test, minutes y minutes Control. 7 3, 4 0.05% NH3 32, 29 80, 52. 64 0.20% NH 30, 30 40, 69,35 1.00% NHL. 37, 35 102,100, 72, 60

In order to determine whether the stabilizing effect of' the ammonia treatment isdue to the y alkality of the ammonia, experiments were con- EXAMPLE II A 64 g. batch of cut cotton was nitrated in a manner similar to that employed in Example I, giving a lot having a nitrogen content of 12.66%. All boiling and beating treatments were made in distilled water and with a Waring blender. At various `stages of the boiling, with or without beating, Samples were removed and each removed sample was divided into twoparts. One part was rinsed and dried and used as the control to indicate stability derived from the boiling and beat.-

ing treatment, the other part being stirred for 15 minutes at room temperature in 1%V aqueous ammonia following the boiling and beating treatments, rinsed and dried.

In the boiling and beating treatment, the water was changed at the end of 4, 16, 28, 40, 50, 63 and hours.

Test results on the removed samples are given in the following table, the asterisks in the table indicating that the samples were too unstable to be tested by the procedure employed, and the :blanks indicating that no test was conducted in the particular instances.

Table Z 134.5 heat test 110 test Boiling and beating treatment Control Atlxla Control Alelzlltrm part part Part part mmutes minutes mmutes minutes None., 20,20 2,2 18,25 Boiled 2 hrs. 4, 4 29, 18 Boiled 4 hrs. 25,25 4, 5 35,30 Boiled 14 hrs 26, 26 4, 5 25, 25 Boiled 2O hrs 5, 5 27, 31 Boiled 30 hrs 8, 7 32, 26 Boiled 40 hrs 15, 17 32, 37 7, 7 23, 17 Boiled 40 hrs. and

beaten 26, 24 23, 23 27, 23 15, 23, 20 Boiled 50 hrs. (beaten at 40 hrs.) 23 16, 22 Boiled 63 hrs. and

beaten (2d time) 27, 29 40, 40 20,23 32, 30 Boiled 70 hrs. (beaten at 40 and 63 hrs.) 24, 24 24, 24 16, 14 23, 23

Comparison cf results given in the data of Table 2 shows that ammonia treatment produces a greater change in stability inthe earlier stage of the boiling and beating treatments than in the Y later stages. A product of high stability is obtained if ammonia is used with only 4 hours prior boil, whereas without the use of ammonia a 40- hour .boil is required to give a corresponding degree of stability. In the examples given above, the ammonia treatment was eifected by using dilute aqueous ammonium hydroxide as the stabilizing agent. However, other ammonium agents, such as ammonium sulfate, may be used. Forexample, freshly prepared nitrocellulose may be boiled with a slightly acid solution containing ammonium salts buffered to remain at a pI-I higher than 3.5, preferably higher than 4, with excellent stabilizing results. In this case, the concentration of ammonium salts in the solution may be as low as 0.10%, calculated as ammonium sulfate. The buffering agent may be any which will give a satisfactory buffering action in the faintly acid region. Sodium acetate', potassium acid phthalate, and sodium citrate have been found satisfactory. One hour lboiling in such a solution gives a nitrocellulose of excellent stability.

Cut cotton fibers were used in the preceding examples. The treatment with cotton linters gives similar stabilization results, as illustrated in the following example:

EXAMPLE III A batch of nitrocellulose prepared from cotton linters and having a nitrogen content of 12.73% was divided into two parts. One part was boiled for 4 hours without ammonia treatmentV and the other was placed in water, adjusted to slight alkalinity with ammonium hydroxide, and stirred for 1 hour at room temperature.

Test results on the two parts are shown in Tableu 3.

As before stated, the action of the ammonia is not due alone to its alkalinity, as has been amply demonstrated by substitution of other bases.

vAlthough the true action is not definitely known,

it may be stated that it is probably due to an ability of the ammonia to penetrate the structure of the nitrocellulose fiber and neutralize thesulfate bound within the fber structure or within the molecules themselves. Experiments relative to sulfate contents and consumed ammonia content tend to exhibit this. Such experiments also indicate the consumption of ammonia in the process and optimum conditions relative to other factors, and they give an indication of the nature of the sulfate association with the nitrocellulose structure.

Quantitative analyses of sulfate and ammonia in nitrocellulose present numerous difficulties. The amounts of sulfate to be determined lie between 0.1 mg. and 8.0 mg. S04 per g. of nitrocellulose, .and the ammonia contents vary between 0.03 mg. and 2.0 mg. NI-Ls per g. The analyses which involve the destruction of the nitrocellulose by chemical digestion are not trustworthy under these conditions, as they introduce complicating factors obscuring the desired results when dealing with such small percentages of sulfate and ammonia.

A substitute procedure involving mechanical separation of the sulfate and ammonia was used, therefore, in which comparatively large samples are given repeated boiling in pure water or in a dilute aqueous solution of HCl and beating treatments.

position products from the cellulose nitrate. By

' using suitably large samples and by concentrating the extracts, it is possible to analyze for the very small traces of sulfate and ammonia removed from the nitrocellulose which probably escape detection in the usual methods of analysis.

The vigorous beating action desired in this method of analysis is well obtained by use of a mination or each may be investigated separately and the amounts totaled for determination of the total sulfate content. Determination of the extracted sulfate is readily accomplished by the turbidimetric method of` Treon and Crutchfield (Ind. Eng. Chem., anal. (1942) ed. 14, p. 119) modied to allow for use of smaller volumes of solutions. With a 25 m1,'volume, amounts of S04 between 011mg. and 2.0 mg. may be determined with considerable accuracy. Observations may be made in a rColeman Model 11 spectrophotometer at a wave length of 540 mu and the values read from a standard curve established with known solutions of sulfuric acid.

In determining the ammonia content of the In this manner, extracts are obtainedY which contain a minimum of interfering decomnitrocellulose, essentially the same procedure of extraction may be followed as in analysis for the sulfate. In this case, it is better to use the 0.01` normal I-ICl solution for the boiling. The removed ammonia may then be determined by use of Nesslers reagent (see formula of Bock and Benedict, as given by Hawk and Bergeim, Practical Physiological Chemistry (1937),` 111th ed., Blakiston, Philadelphia, Pennsylvania, p. 928) and the blank and test samples compared with the .Coleman spectrophotometer at 425 mu, the quan-V tity of ammonia being read from a standard curve established with known solutions of ammonium chloride. i g

By using the 0.01 normal HC1 solution for boiling, both the sulfate and ammonia determina-1 tions may be made simultaneously on the same sample. V f

In analysis for the ammonia according to the above-outlined procedure, ithas been found thaty repeated boiling with changes of solution does not show the amount of ammonia removed in successive extracts to approach zero, as would be expected. Rather, the readings approach a minimum of approximately 0.03 mg. NH3 per g. of nitrocellulose. This minimum is usually reached after four boiling treatments.

Testing the analysis on treatments withboiling and beating, using many samples of nitrocellulose which had not been exposed to an ammonia treatment and were presumably free from ammonia, showed an average of 0.034 mg. NH3 (or at least a substance which reacted with Nesslers reagent), per g. of nitrocellulose.` Therefore, it has been concluded that the observed minimum of approximately 0.03 mg. NH3 per g. of nitrocellulose is a systematic error in the procedure and all values are adjusted by subtracting 0.034 from the observed value in each separate reading.

It has also been found that the vigorous beating given the nitrocellulose in the above-described manner of mechanical separation of the sulfate, quickly liberates the sulfate even from samples which had been subjected to long boiling, and that a short period of locating (5 minutes) is accompanied by an increase in stability of the nitrocellulose, provided the nitrocellulose has already been extensively boiled. However, continued beating introduces unstability. Therefore, it appears that the beating results in two competing factors: first, stabilizing due to separation of the sulfate from the nitrocellulose, accomplished in a short period; and second, a harmful effect probably due to some action of the beater on the nitrocellulose itself, introducing unstability in the longer periods of beating. For this reason, unless otherwise noted, all described beating periods are for 5 minutes.

By using the extraction method of analysis for the sulfate and by testing the nitrocellulose for stability at various stages in the extracting process, precise information on the effect of sulfate on stability was obtained. As' representativaa 20 g. sample of nitrocellulose (13.0% N) 4was boiled with changes of distilled water, beating treatments being introduced at intervals, and the several extracts were analyzed for S04, the processbeing continued until a negligible amount ofA SOiwas'removed in the final' treatments.- By computing the amount of 'sulfate removed in each extract per gram of sample and adding the re sults, the total sulfate content of the original sample was obtained. Also, the sulfate content of the sample at the beginning of any stage of removal was similarly determined by adding'the,

S04 .determinations of theextracts including that stage and those following. Table 4 indicates the results of lone such experiment.

8 lulose (12.82% N), treated only by rinsing, whichwas then stirred for 15 minutes in a 1% aqueous ammonia solution and washed with distilled water until the washings gave no test with Nesslers re-` ,Tabfz 4 5 agent. This sample had a stability of 45 minutes T H 1f t'i with the 110 test, as compared to2 minutes in f 'rated o aoilii e stability the absence of the ammonia treatment.. ,.Ten.v f Treatment boil, Ssmpeat 11Qiest grams of the ammonia-treated sample was re,-

hms eigf'lgg muws i 'peaiediy boiled with 25o m1. `ef aornermai HC1, l and the extracts obtained from each boiling by Rinsednu- ,V 0 .L05 ltering on washed paper were made up to known goile? rsg volume for determination of .sulfate andammo-V Bghd g 23 0184 '4 nia contents. Toward the end of the boiling, .a ggg lrhlgz g beating treatment was introduced. The amountsA eaten mm 0125 12 l5 of sulfate'and ammonia removed fromthe sam- Boiled ,7,1m 53 0-20 15 ple by the various extracts are indicated in Table Blllf, gjg gjgg 5, all ammonia Values being adjusted by subggg hrlsgg 8- 16 traction of the 0,34 mg./g. to compensate for the Boiled 1c 135111111111 97 -olia systematic error before described. Boiled 58 hrs. with 8 changes f 20 ci water and beaten 5mm.k 155 0.00 Table 6 The relationship of sulfate content and sta- Y Ammonia' Sulfateo) 4 re- 4 bility 1s morel easlly vlsualized by reference to Boiling treatment moved removed, l curve A shown in Figure 2 of the drawing, in 25 ing/g. mgJg' which the ordinate represents S04 content in mg. per g. of nitrocellulose, and the abscissa rep- 1st extract (boiled 14 hrs.) 1-08 4.35 resents stability as determined by the 110 test iitiitiiiii: It? 13% the curve indicating less stability after the 4th extract (boiled2hrs.) V.00 .15 minute point, since unstability beyond that point 30 fggillgfgf; is introduced due to the beating treatment. -01 .18 Curve A dirigere 2 indicates that stability is '0' m a function' of S04 content, provided other fac- Total ,1-40 k6.43 tors, such as excessive beating, are not introduced V which tend to alter the stability. The data of Table 6 show that as` regards the If ammonia treatment is given, the Stability sample tested, 1.4 mg. of ammonia is suicient to results are entirely different, as indicated in Tastabilize l g. of freshly prepared nitrocellulose ble 5 showing tests on a number of samples of containing 6.43 mg. of sulfate. This gives aratio nitrocellulose having dilerent S04 contents, and of mols of NH3 to Inols of S04 oi' 1.20. treated for l5 minutes at room temperature with 40 A number of otherexperiments similar to thel a'1% ammonia solution. one described next above were carried out with Table 5 other samples, some without and some with the boiling treatment, the results of which are shown SO tent 1% ammqnia g'labble 7, firts lirlie beiig the total ling of C011 treatment V.a e an e o i er ines eing correspon ing V Pmwustreatmentufsamples Digg/21,65 taity 10 lines for the other samples, a column for the es mm es ratio of meis ei NH3 te mois di S04 being added.

mused free ofaeids .4.64 1415,18 Table 7 Boiled 30 min-.. 3.20 28 Boiled 2 hrs 1.40 25, 25 Boiled/8ms.. s .67 35,45 5() Total Total gglg ller: fg, Treatment pil'leceding stabilizaamrmia sulfate Mol NH.; eeeeeeaehaitiana-attimi: :di 15118 m W 1% mmm (ILL/3g); tig/4g); M015@ The results of Table 5 are visually represented Rinsed (from Table 6). 1.40 6.43 1.20 in curve B of Figure 2. This curve shows that filsegfjm* jg lj good stability is achieved by the use of the am- B0i1ed2hrs .37 1.40 L49 momia treatment with nitrocellulose containing as high as 3.2 mg. S04 per g. of nitrocellulose. As The data of Table l show toc wide a variation the S04 content approaches zero, curves A and to rbe taken as establishing an exact vrelationship B tend to meet, indicating that the effect of the 00 between the ammonia consumed in the treatment ammonia treatment ls to oiset the unstability due and the sulfate present in the sample.` However, to presenceof the S04. Also, apparently the arnin the tests for determination of ammoniaQthe monia treatment has no elect on other factors ammonia content in the sample will include not causing'unstability. only that consumed in neutralizing the unstabili- Again, it is mentioned that the stability ob- U5 zation dueto the S04, but probably in addition -a tained by the ammonia treatment is not due, solesmall amount bound' by the nitrocellulose within ly atleast, to its alkalinity, since tests with other the ber structure, held by liquid retained on the portions of the same samples used in obtaining treated ber, o r both. It isprobable, therefore, the data for Table 5, but substituting a sodium that the ratios are higher than the ratios required hydroxide treatment for the same ammonia for neutralization of the S04 itself. treatment, gave no noticeable increasein stability If the sulfate is present in the nitrocellulose over the samples not so treated; ber as half-ester represented by R-S04H as has The marmer `by which ammonia brings about been suggested, l mol of NH3 would combine With stabilization of nitrocellulose is indicated in the l mol of S04. If, however, the sulfate is.l present' following experiment using a sample of nitrocel- 25 as free acid, as has been suggested'also, 2 molspf 9 NH3 Would combine with 1 mol of S04. Therefore, the data above favor the half-ester suggestion.

In another experiment, freshly prepared nitrocellulose (12.6% N) was treated with ammonia, rinsed and dried, giving a resulting stability with the 110 test of 18 minutes. Three grams of the stabilized material was then stirred for 10 minutes with 75 ml. of 5 normal HCl, rinsed free from Cl ions, and dried, resulting in a nitrocellulose reduced in stability to minutes, which could not be improved by further rinsing. However, upon a retreatment with ammonia, good stability again obtained.

The evidence at hand as shown by the foregoing experiments indicates that the ammonia treatment oisets the harmful eifect of the sul furic acid bound within the structure or fiber of the nitrocellulose, and that other bases, at least the many tried, are unable to accomplish this result, possibly because of inability to penetrate to the position where the sulfate is iixed.

It may be noted from the experiments that a very small amount of ammonia actually is involved in the stabilization of the nitrocellulose, 1.40 mg. of NH3 per g. of nitrocellulose being indicated in Table 6. It has been shown that with nitrocellulose which has been boiled for 4 hours and beaten, less than 0.1 mg. per g. is required. These amounts are almost negligible.

In actual production of the stabilized nitrocellulose, the ammonia consumption would include also the small amount bound by the nitrocellulose and any lost by the liquid retained on the ber when draining the treating solution from the nitrocellulose. The liquid loss involved in this latter amount, without taking special precautions for recovery, may be roughly ten times the weight of the nitrocellulose ber treated. Thiswould involve a loss of 5 pounds of ammonia per 1,000 pounds of nitrocellulose on the basis of employment of a 0.05% ammonia solution. Under these conditions, the total loss of ammonia would approximate 0.64% of the weight of the fiber treated, and this could be considerably decreased by pressing or centrifuging the treated fiber to recover the retained liquid or by recovery of the ammonia from the water used in washing the retained liquid from the ber.

Since the ammonia solution drained ou' the batch of treated nitrocellulose may be reused, if desired, in treating other batches, and since the process is attended with little work and. is accomplished in a short time, it may be seen that the invention involves a very simple and inexpensive manner of treatment.

The above examples and experiments were all conducted with laboratory-prepared samples and equipment. In the following example, the nitrocellulose sample was from a batch being prepared for `making pyro powder.

EXAMPLE IV 1.5 pounds of nitrocellulose were taken from a boiling tub following two 4-hour periods of boiling with changes of water. The sample was beaten in a laboratory beater to neness approximating that of nished pyro. It was next boiled for four hours and allowed to settle. The water was decanted, replaced with cold water to which was added aqueous ammonia containing NH3 amounting to 1 percent of the weight of the nitrocellulose. The suspension was stirred at room temperature for 1 hour, allowedto settle, and washed by decantation with eight changes of water. The product gave stability of 29 minutes by the 134.5 German test and 37 minutes by the 65.5 starch-potassium iodide test (this test is a standard procedure described in War Department Technical Manual 'IM 9-29900). Similar stable products were produced with 6.5 to 15 hours of boiling.

It is desirable that a certain amount of boiling follow the beating treatment. this example was a. small paper mill type beater. Excellent stability was also produced when ammonia was introduced into the solution during the beating operation. This has the advantage of reducing acidity and hence reducing the corrosion of the beater as Well as produing good stability.

Having thus described the invention, what is claimed is:

1. A process comprising stirring nitrocellulose in an aqueous solution containing ammoniaat room temperature for a time not to exceed about l hour, thereby to stabilize the nitrocellulose.

2. A process comprising stirring nitrocellulose with weak ammonium hydroxide at room temperature for a time not to exceed aboutl 1 hour, thereby to stabilize the nitrocellulose.

RICHARD E. REEVES. JOEL E. GIDDENS.

The beater inv 

